It has been demonstrated that changes in ionic permeability of the endolymph/perilymph barrier are a significant factor in the hearing loss produced by noise or ototoxic antibiotics. The present study has examined the physiological effects on the cochlea of two drugs, amiloride and tetraethylammonium (TEA) which are reported to reduce membrane permeabilities to sodium and potassium ions respectively in a variety of tissues. The application of TEA to coclear endolymph produced effect similar to noise exposure. Cochlear microphonics (CM) were reduced, endocochlear potential (EP) was increased and the rate of EP decline during anoxia decreased. Application of amiloride to the cochlea did not affect CM or EP. These results support the view that normal potassium permeability properties of the endolymph/perilymph barrier are essential for normal cochlear function.